(a) Field of the Invention
The present invention relates to a block artifact phenomenon eliminating device of a decoded image frame and an eliminating method thereof.
(b) Description of the Related Art
In most conventional image data encoding schemes, an image frame is processed block by block. In particular, according to encoding standards of image data such as MPEG (moving picture experts group), H.264, etc., image data is encoded based on blocks. In other words, in an encoding process of an image frame, a frame is divided into several blocks, and data processes are respectively performed based on each block. Here, blocking artifacts caused by a contour line of a block boundary may occur. This blocking artifact brings about image quality degradation and a user's visual discomfort.
This block artifact phenomenon is caused by a loss of low frequency band information occurring in encoding and quantization processes in which correlation between blocks is not considered. In more detail, a high frequency region (i.e., edge region), which does not exist in an original image frame that is not compressed, occurs in an image frame which is realized through encoding and decoding processes. The block artifact phenomenon destroys continuity among adjacent pixels. To prevent the block artifact phenomenon, a deblocking process, which eliminates the block artifact phenomenon by smoothing the high frequency region with a low pass filter, has been utilized.
In addition to the deblocking process using the low pass filter, another method for eliminating the block artifact phenomenon has been proposed. According to the method, pixels in a block are divided into inner block pixels and block boundary pixels, and low pass filtering strength is controlled according to the divided pixels. In addition, another low pass filtering method has been proposed, and according to the method, a set of pixels in which a difference between pixel values is larger than a predetermined threshold is selected among pixels neighboring a block boundary, and a predetermined virtual image frame is properly inserted therein in order to offset the pixel value difference around the block boundary. Accordingly, images may be controlled to be smooth around the block boundary, and visual discontinuity may be reduced.
However, even though the above-described conventional block artifact phenomenon eliminating methods can reduce the visual discontinuity occurring around the block boundary, the method also smoothes the high frequency region data (i.e., object edge) corresponding to contour lines of a person or an object located around the block boundary region. Accordingly, a problem that image quality is degraded occurs.
Therefore, it is required to determine accurately where the block artifact phenomenon occurs in an image in order to perform filtering for block artifact elimination. Otherwise, an edge region existing in the original image frame may be eliminated. In other words, the object edge may be mis-determined to be a block artifact, and may be eliminated. Consequently, image quality can be rather degraded after filtering.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.